1. Field of the Invention
The present invention relates to an adaptive communication apparatus used for improving communications quality of a multicarrier transmission system in which data are transmitted through use of a plurality of carriers allocated various frequencies.
2. Description of the Related Art
A multicarrier transmission has been proposed as a radio communication method for transmitting a large amount of data at high speed.
The multicarrier transmission method will be described with reference to FIG. 10. At the transmitting side, by a serial-to-parallel converter 206, a data sequence to be transmitted is divided into a plurality of low-speed data sequences which have a lower-speed transmission rate than that of the data sequence to be transmitted. The thus-divided low-speed data sequences are modulated by modulators 205 and converted into high-frequency signals allocated various carrier frequencies. By a combiner 203, these high-frequency signals are combined into a so-called multicarrier signal comprising a plurality of carrier signals, and the multicarrier signal is radiated from an antenna 201.
At the receiving side, the transmitted data are reproduced by performing the reverse of the foregoing operation. More specifically, the multicarrier signal received by an antenna 1 is subjected to wave separation by a wave separator 3 in order to obtain a plurality of carriers. The thus-separated respective carriers are demodulated by demodulators 5 to obtain low-speed data sequences. By a parallel-to-serial converter 6, the low-speed data sequences are converted into a high-speed transmission data sequence, thereby yielding a received data sequence.
In general, the frequency characteristic of transmission between the antenna on the transmitting side and the antenna on the receiving side (hereinafter referred to as the "channel frequency-characteristic") is distorted in a so-called multi-path environment in which undesired interference waves arrive at the antenna along with a desired incident wave. In a case where the bandwidth of a transmitted signal is narrow, and the channel frequency-characteristic within this bandwidth can be regarded as flat, the spectrum and waveform of a received signal are analogous to those of the transmitted signal. If a receiver moves in such a case, the spectrum within the band varies uniformly. Such a variation of the spectrum is called a flat fading.
In contrast, in a case where the bandwidth of the transmitted signal is broad, and the channel frequency-characteristic within the bandwidth is distorted, the spectrum and waveform of the received signal is not analogous to that of the transmitted signal. Specifically, the spectrum of the signal received by the receiving antenna is the product of the spectrum of the transmitted signal and the channel frequency-characteristic. The spectral component of the transmitted signal is not received at the frequency at which there exists much fade in the channel frequency-characteristic. The fading in which a receiving level fades only at a specific frequency is called a frequency-selective fading. In case of a frequency-selective fading, the waveform of the received signal is severely distorted, thereby considerably deteriorating communications quality.
The bandwidth of a signal to be transmitted depends on the transmission rate of the data. The higher the transmission rate of the data, the wider the bandwidth of the transmitted signal becomes. Accordingly, as data are transmitted at higher speed, deterioration of communications quality attributable to the frequency-selective fading becomes more severe.
In the multicarrier transmission system, a transmitted data sequence is divided into a plurality of data sequences, and the respective data sequences are converted into a high-frequency signal that includes a plurality of carriers having different frequencies. More specifically, the influence of the frequency-selective fading is alleviated by reducing the data transmission rate of each carrier in order to narrow the bandwidth of each carrier. Accordingly, the multicarrier transmission method is expected to serve as a technique for realizing high-speed and high-quality communications.
However, the reduction of the receiving level owing to the flat fading arises even if the data transmission rate per carrier is reduced so as to prevent the deterioration of communication quality attributable to the influence of the frequency-selective fading. The plurality of carriers may include a carrier whose receiving level becomes low due to the flat fading. In this case, the data transmitted through use of such a carrier are not correctly demodulated at the receiving side.
To prevent this problem, there has been conceived a method in which if a multicarrier signal includes a carrier which cannot be correctly demodulated, the data of this carrier signal is corrected through use of correctly-demodulated data of other carrier signals. In the proposed method, however, data used for determining whether or not the data have been correctly received or data used for correcting the erroneous data must be added to the original data to be transmitted. As a result, in comparison with the case having no such redundant data, the amount of information transmitted per unit bandwidth is reduced, thereby resulting in a reduction in frequency utilization efficiency.
Another countermeasure against this problem is described in Japanese Patent Application Laid-Open (kokai) No. 7-66739, in which variation in the amplitude of each received carrier with time is detected, and the frequency characteristic of the carrier signal is corrected so as to reduce the variation. FIG. 11 shows an embodiment of this method. According to the conventional technique shown in FIG. 11, the signals received by a plurality of antennas 1 are separated by wave separators 3 in order to obtain a plurality of paired carriers. The phase of one of the paired carriers is controlled based on a minimum amplitude deviation diversity scheme, and the paired carrier signals are combined together. More specifically, an amplitude deviation detector 101 detects the amplitude deviation of the combined signal, and a signal representing the thus-detected amplitude deviation is input to a control circuit 102. The control circuit 102 controls a phase shifter 103 so as to minimize the thus-detected amplitude deviation. Demodulator 5 demodulates the combined carriers to obtain corresponding data sequences, and the data sequences are converted into the original data by a parallel-to-serial converter 6.
However, if the paired carriers are combined together through use of the minimum amplitude deviation diversity, there is a possibility that the amplitude of the combined signal becomes small, depending on a phase difference between a desired wave and an interference wave. Therefore, the foregoing problem, i.e., the deterioration of communication quality of a specific carrier, cannot be solved. To prevent this situation, the conventional communication apparatus is provided with a carrier determination circuit 104 and a plurality of control circuits 102. If one of the control circuits 102 for a certain carrier cannot properly control the phase of the certain carrier because the amplitude of the corresponding combined signal is too small, the carrier determination circuit 104 produces control information for the certain carrier on the basis of information from other control circuits which properly perform phase control, and sends the thus-prepared control information to the control circuit 102 for the certain carrier.
In this case, however, the other control circuits are also controlled based on the minimum amplitude deviation diversity. Even if the amplitude deviation of the certain carrier is reduced as a result of controlling the phase of the corresponding counterpart carrier on the basis of the information from other control circuits, there is no guarantee that the receiving level of the certain carrier increases. Accordingly, the foregoing problem, the deterioration of communication quality of a specific carrier, cannot be solved comprehensively.
As described above, the influence of the frequency-selective fading can be reduced by the multicarrier transmission method. However, the communication quality of a certain carrier may be deteriorated due to the flat fading. This problem is solved by the method of adding redundant data to the original data to be transmitted, which, however, results in a reduction in the frequency utilization efficiency of the overall multicarrier signal. Further, as previously described, the method employing the minimum amplitude deviation technique cannot be said to constitute means for completely eliminating the deterioration of the communication quality of a certain carrier. For these reasons, there is demand for the development of a adaptive communication apparatus for use in a multicarrier transmission system which is capable of reliably demodulating all the data of carriers without sacrificing frequency utilization efficiency.